Larson was born Brianne Sidonie Desaulniers on October 1, 1989, in Sacramento, California, to Heather (Edwards) and Sylvain Desaulniers. Her parents were homeopathic chiropractors who ran a practice together, and they have another daughter, Milaine. Her father is French Canadian, with roots in Winnipeg, and in her childhood, Larson spoke French as her first language. She was mostly homeschooled, which she believed allowed her to explore innovative and abstract experiences. Describing her early life, Larson has said that she was "straight-laced and square", and that she shared a close bond with her mother but was shy and suffered from social anxiety. During the summer, she would write and direct her own home movies in which she cast her cousins and filmed in her garage. At age six, she expressed interest in becoming an actress, later remarking that the "creative arts was just something that was always in me". That same year, she auditioned for a training program at the American Conservatory Theater in San Francisco, where she became the youngest student admitted.